Flint is an impure brown chalcedony, usually forming concretions. The color is due to organic matter. Flint is mostly found in limestone or chalk, and the concretions are the result of the small particles of silica scattered through the rock being dissolved, and then reprecipitated about some organic center. Generally the silica was obtained by the dissolution of small fossils, like the shells of diatoms or sponge spicules. Hornstone and Chert are simply impure varieties of flint, brown in color, and with a splintery fracture. Agate, Plate 32, is a banded or cloudy chalcedony which has formed in a cavity, the layers of different color representing deposition from water, carrying first silica with one impurity, then later, silica with another impurity. Gradually the cavity has been thus filled with silica; and when the mass is freed by the weathering away of the surrounding rock, these banded masses are found. Sometimes the manner of deposition has changed, and while the outer part of the cavity was filled with chalcedony, the central part will contain quartz crystals. On account of the beauty of the colors, and the unusual way in which they may be developed, agates are widely used for semiprecious jewelry and objects of art, and this has been true since ancient times, the name itself coming from the River Achates in Sicily. The center for cutting and polishing agates is at Oberstein, Germany, where this work has been carried on since the middle of the fifteenth century. In spite of the many fine natural colors in agates, they are sometimes artificially colored, in many cases by methods which are kept as “trade secrets.” The color seldom penetrates far; so that even slight chipping reveals whether an inferior agate has been taken and colored up, or whether the stone is natural. Moss agates are chalcedony which has inclosed dendritic masses of some one of the manganese compounds as shown under manganite, p. 73. Onyx is a variety of agate where the bands are alternately black and white; while sardonyx is agate with red or brown bands alternating with the white. Such agates as these are especially desirable for cameo work, where the figure is carved in the chalcedony of one color, and the other color makes the background. Silicified or _agatized wood_ is a form of chalcedony, where silica has replaced wood, molecule by molecule; so that in good specimens, all the structure of the wood is still retained, and when thin sections are made it can be studied under the microscope almost as well as modern wood. This takes place under water, usually, if not always, in fresh water. Such fossilized wood is widely distributed in the western United States, the most famous cases being the Fossil Forest of Arizona, now a National Reservation, and the fossil trees in the Yellowstone National Park. Opal SiO₂·H₂O Pl. 33